Nowadays several medical image acquisition techniques and systems exist that render a digital signal representation of a medical image, e.g. a radiographic image.
One example of such a system is a computed radiography system wherein a radiation image is recorded on a temporary storage medium, more particularly a photostimulable phosphor screen. In such a system a digital signal representation is obtained by scanning the screen with radiation of (a) wavelength(s) within the stimulating wavelength range of the phosphor and by detecting the light emitted by the phosphor upon stimulation.
Other examples of computed radiography systems are direct radiography systems, for example systems wherein a radiographic image is recorded in a solid state sensor comprising a radiation sensitive layer and a layer of electronic read out circuitry.
Still another example of a computed radiography system is a system wherein a radiographic image is recorded on a conventional x-ray film and wherein that film is developed and subsequently subjected to image scanning.
Still other systems such as a tomography system may be envisaged.
The digital image representation of the medical image acquired by one of the above systems can then be used for generating a visible image on which the diagnosis can be performed. For this purpose the digital signal representation is applied to a hard copy recorder or to a display device.
Commonly the digital signal representation of the image is subjected to image processing prior to hard copy recording or display.
In order to convert the digital image information optimally into a visible image on a medium on which the diagnosis is performed, a multi-scale image processing method has been developed by means of which the contrast of an image is enhanced.
According to this multi-scale image processing method an image represented by an array of pixel values is processed by applying the following steps. First the original image is decomposed into a sequence of detail images at multiple scales and a residual image.
Next, the pixel values of the detail images are modified by applying to these pixel values at least one non-linear monotonically increasing odd conversion function with a slope that gradually decreases with increasing argument values. In a specific embodiment the slope of the conversion function is gradually decreasing with the exception that it may be constant or increasing in a lower subrange which is assumed to represent mostly noise.
Finally, a processed image is computed by applying a reconstruction algorithm to the residual image and the modified detail images, the reconstruction algorithm being such that if it were applied to the residual image and the detail images without modification, then the original image or a close approximation thereof would be obtained.
The above image processing technique has been described extensively in European patent EP 527 525, the processing being referred to as MUSICA image processing (MUSICA is a registered trade name of Agfa-Gevaert N.V.).
The described method is advantageous over conventional image processing techniques such as unsharp masking etc. because it increases the visibility of subtle details in the image and because it increases the faithfulness of the image reproduction without introducing artefacts.
However, although the method disclosed in EP 527 525 is able to selectively enhance contrast in a predefined intermediate subrange which is considered most critical the method has a disadvantage in that by imposing the constraint of decreasing slope at least in the upper subrange, the amount of selective amplification is too much limited.
In the article by Fivez C. et al. entitled “Multi-resolution contrast amplification in digital radiography with compensation for scattered radiation” published in Proceedings of International Conference on Image Porcessing, Lausanne, Sep. 16-19, 1996, New York, IEEE, US, Vol. 1, p. 339-342, the above method is likewise described.
In European patent application EP 0 574 969 a noise reducing method has been disclosed. The noise reduction method may be followed by an additional process of boosting image contrast. The enhanced contrast may be obtained by modifying pixel values of noise reduced detail images according to a non-linear monotonically increasing odd mapping function with a slope that gradually decreases with increasing argument before supplying the modified detail images to a reconstruction section.
European patent application EP 0 971 316 relates to a method wherein a digital image representation is decomposed into detail, images at successive resolution levels.
The decomposed images are modified during an enhancement phase by applying a method which is different from a amendment of the detail images by applying to the pixel values a non-linear monotonically increasing odd conversion function. According to the method of EP 971 316 pixels of the detail images are pixel-wise multiplied with gain images comprising gain factors for each individual pixel of a detail image.
During a reconstruction process the decomposed images are recombined after application of the gain images.